ABFAB J. Howlett
Internet-Draft JANET(UK)
Intended status: Informational S. Hartman
Expires: January 30, 2012 Painless Security
H. Tschofenig
Nokia Siemens Networks
E. Lear
Cisco Systems GmbH
July 29, 2011
Application Bridging for Federated Access Beyond Web (ABFAB)Architecturedraft-ietf-abfab-arch-00.txt
Abstract
Over the last decade a substantial amount of work has occurred in the
space of federated access management. Most of this effort has
focused on two use-cases: network and web-based access. However, the
solutions to these use-cases that have been proposed and deployed
tend to have few common building blocks in common.
This memo describes an architecture that makes use of extensions to
the commonly used security mechanisms for both federated and non-
federated access management, including RADIUS, Diameter, GSS, GS2,
EAP and SAML. The architecture addresses the problem of federated
access management to primarily non-web-based services, in a manner
that will scale to large numbers of identity providers, relying
parties, and federations.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 30, 2012.
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Copyright Notice
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document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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described in the Simplified BSD License.
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Internet-Draft ABFAB Architecture July 20111. Introduction
The Internet uses numerous security mechanisms to manage access to
various resources. These mechanisms have been generalized and scaled
over the last decade through mechanisms such as SASL/GS2 [RFC5801],
Security Assertion Markup Language (SAML) [OASIS.saml-core-2.0-os],
RADIUS [RFC2865], and Diameter [RFC3588].
A Relying Party (RP) is the entity that manages access to some
resource. The actor that is requesting access to that resource is
often described as the Subject. Many security mechanisms are
manifested as an exchange of information between these actors. The
RP is therefore able to decide whether the Subject is authorised, or
not.
Some security mechanisms allow the RP to delegate aspects of the
access management decision to an actor called the Identity Provider
(IdP). This delegation requires technical signalling, trust and a
common understanding of semantics between the RP and IdP. These
aspects are generally managed within a relationship known as a
'federation'. This style of access management is accordingly
described as 'federated access management'.
Federated access management has evolved over the last decade through
such standards as SAML [OASIS.saml-core-2.0-os], OpenID [1], and
OAuth [RFC5849], [I-D.ietf-oauth-v2]. The benefits of federated
access management include:
Single or Simplified sign-on:
An Internet service can delegate access management, and the
associated responsibilities such as identity management and
credentialing, to an organisation that already has a long-term
relationship with the Subject. This is often attractive for
Relying Parties who frequently do not want these responsibilities.
The Subject may also therefore require fewer credentials, which is
often desirable.
Privacy:
Often a Relying Party does not need to know the identity of a
Subject to reach an access management decision. It is frequently
only necessary for the Relying Party to establish, for example,
that the Subject is affiliated with a particular organisation or
has a certain role or entitlement. Sometimes the RP does require
an identifier for the Subject (for example, so that it can
recognise the Subject subsequently); in this case, it is common
practise for the IdP to only release a pseudonym that is specific
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to that particular Relying Party. Federated access management
therefore provides various strategies for protecting the Subject's
privacy. Other privacy aspects typically of concern are the
policy for releasing personal data about the Subjectfrom the IdP
to the RP, the purpose of the usage, the retention period of the
data, and many more.
Provisioning
Sometimes a Relying Party needs, or would like, to know more about
a subject that an affiliation or pseudonym. For example, a
Relying Party may want the Subject's email address or name. Some
federated access management technologies provide the ability for
the IdP to provision this information, either on request by by the
RP or unsolicited.
1.1. Terminology
This document uses identity management and privacy terminology from
[I-D.hansen-privacy-terminology]. In particular, this document uses
the terms pseudonymity, unlinkability, anonymity, and identity
management.
We make one note about the IdP: in this architecture, the IdP
consists of the following components: an EAP server, a radius server,
and optionally a SAML Assertion service. The IdP is also responsible
for authentication, even though it may rely upon other components
within a domain for such an operation. The reader is advised that
for succinctness, in most cases the term IdP is used, except where
additional clarity seems appropriate.
1.2. An Overview of Federation
In the previous section we introduced the following actors:
o the Subject,
o the Identity Provider, and
o the Relying Party.
These entities and their relationships are illustrated graphically in
Figure 1.
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,----------\ ,---------\
| Identity | Federation | Relying |
| Provider + <-------------------> + Party |
`----------' '---------'
<
\
\ Identity
\ management
\
\
\
\ +------------+
\ | |
v| Subject |
| |
+------------+
Figure 1: General federation framework model
A federation typically this relationship encompasses operational
specifications and legal rules:
Operational Specifications:
This includes the technical specifications (e.g. protocols used to
communicate between the three parties), process standards,
policies, identity proofing, credential and authentication
algorithm requirements, performance requirements, assessment and
audit criteria, etc. The goal of these specifications to make the
system work and to accomplish interoperability.
Legal Rules:
The legal rules take existing laws into consideration and provide
contractual obligations to provide further clarification and
define responsibilities. These legal rules regulate the
operational specifications, make operational specifications
legally binding to the participants, define and govern the rights
and responsibilities of the participants. These legal rules may,
for example, describe liability for losses, termination rights,
enforcement mechanisms, measures of damage, dispute resolution,
warranties, etc.
The nature of federation dictates that there is some form of
relationship between the identity provider and the relying party.
This is particularly important when the relying party wants to use
information obtained from the identity provider for access management
decisions and when the identity provider does not want to release
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information to every relying party (or only under certain
conditions).
While it is possible to have a bilateral agreement between every IdP
and every RP; on an Internet scale this setup requires the
introduction of the multi-lateral federation concept, as the
management of such pair-wise relationships would otherwise prove
burdensome.
While many of the non-technical aspects of federation, such as
business practices and legal arrangements, are outside the scope of
the IETF they still impact the architectural setup on how to ensure
the dynamic establishment of trust.
Some deployments are sometimes required to deploy complex technical
infrastructure, including message routing intermediaries, to offer
the required technical functionality, while in other deployments
those are missing.
Figure 1 also shows the relationship between the IdP and the Subject.
Often a real world entity is associated with the Subject; for
example, a person or some software.
The IdP will typically have a long-term relationship with the
Subject. This relationship would typically involve the IdP
positively identifying and credentialling the Subject (for example,
at time of enrollment in the context of employment within an
organisation). The relationship will often be instantiated within an
agreement between the IdP and the Subject (for example, within an
employment contract or terms of use that stipulates the appropriate
use of credentials and so forth).
While federation is often discussed within the context of relatively
formal relationships, such as between an enterprise and an employee
or a government and a citizen, federation does not in any way require
this; nor, indeed, does it require any particular level of formality.
It is, for example, entirely compatible with a relationship between
the IdP and Subject that is only as weak as completing a web form and
confirming the verification email.
However, the nature and quality of the relationship between the
Subject and the IdP is an important contributor to the level of trust
that an RP may attribute to an assertion describing a Subject made by
an IdP. This is sometimes described as the Level of Assurance.
Similarly it is also important to note that, in the general case,
there is no requirement of a long-term relationship betweem the RP
and the Subject. This is a property of federation that yields many
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of its benefits. However, federation does not preclude the
possibility relationship between the RP and the Subject, should needs
dictate.
Finally, it is important to reiterate that in some scenarios there
might indeed be a human behind the device denoted as Subject and in
other cases there is no human involved in the actual protocol
execution.
1.3. Challenges to Contemporary Federation
As the number of such federated services has proliferated, however,
the role of the individual has become ambiguous in certain
circumstances. For example, a school might provide online access to
grades to a parent who is also a teacher. She must clearly
distinguish her role upon access. After all, she is probably not
allowed to edit her own child's grades.
Similarly, as the number of federations proliferates, it becomes
increasingly difficult to discover which identity provider a user is
associated with. This is true for both the web and non-web case, but
particularly acute for the latter ans many non-web authentication
systems are not semantically rich enough on their own to allow for
such ambiguities. For instance, in the case of an email provider,
the use of SMTP and IMAP protocols does not on its own provide for a
way to select a federation. However, the building blocks do exist to
add this functionality.
1.4. An Overview of ABFAB-based Federation
The previous section described the general model of federation, and
its the application of federated access management. This section
provides a brief overview of ABFAB in the context of this model.
The steps taken generally in an ABFAB federated authentication/
authorization exchange are as follows:
1. Principal provides NAI to Application: Somehow the client is
configured with at least the realm portion of an NAI, which
represents the IdP to be discovered.
2. Authentication mechanism selection: this is the step necessary
to indicate that the GSS-EAP SASL/GS2 mechanism will be used for
authentication/authorization.
3. Client Application provides NAI to RP: At the conclusion of
mechanism selection the NAI must be provided to the RP for
discovery.
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4. Discovery of federated IdP: This is discussed in detail below.
Either the RP is configured with authorized IdPs, or it makes
use of a federation proxy.
5. Request from Relying Party to IdP: Once the RP knows who the IdP
is, it or its agent will forward RADIUS request that
encapsulates a GSS/EAP access request to an IdP. This may or
may not contain a SAML request as a series of attributes.. At
this stage, the RP will likely have no idea who the principal
is. The RP claims its identity to the IdP in AAA attributes,
and it makes whatever SAML Attribute Requests through a AAA
attribute. XXX- Check order of SAML attribute request.
6. IdP informs the principal of which EAP method to use: The
available and appropriate methods are discussed below in this
memo.
7. A bunch of EAP messages happen between the endpoints: Messages
are exchanged between the principal and the IdP until a result
is determined. The number and content of those messages will
depend on the EAP method. If the IdP is unable to authenticate
the principal, the process concludes here. As part of this
process, the principal will, under protection of EAP, assert the
identity of the RP to which it intends to authenticate.
8. Successful Authentication: At the very least the IdP (its EAP
server) and EAP peer / subject have authenticated one another.
As a result of this step, the subject and the IdP hold two
cryptographic keys- a Master Session Key (MSK), and an Extended
MSK (EMSK). If the asserted identity of the RP by the principal
matches the identity the RP itself asserted, there is some
confidence that the RP is now authenticated to the IdP.
9. Local IdP Policy Check: At this stage, the IdP checks local
policy to determine whether the RP and subject are authorized
for a given transaction/service, and if so, what if any,
attributes will be released to the RP. Additional policy checks
will likely have been made earlier just through the process of
discovery.
10. Response from the IdP to the Relying Party: Once the IdP has
made a determination of whether and how to authenticate or
authorize the principal to the RP, it returns either a negative
AAA result to the RP, or it returns a positive result to the RP,
along with an optional set of AAA attributes associated with the
principal that could include one or more SAML assertions. In
addition, an EAP MSK is returned to the subject.
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Internet-Draft ABFAB Architecture July 20111.5. Design Goals
Our key design goals are as follows:
o Each party of a transaction will be authenticated, and the
principal will be authorized for access to a specific resource .
o Means of authentication is decoupled so as to allow for multiple
authentication methods.
o Hence, the architecture requires no sharing of long term private
keys.
o The system will scale to large numbers of identity providers,
relying parties, and users.
o The system will be designed primarily for non-Web-based
authentication.
o The system will build upon existing standards, components, and
operational practices.
Designing new three party authentication and authorization protocols
is hard and frought with risk of cryptographic flaws. Achieving
widespead deployment is even more difficult. A lot of attention on
federated access has been devoted to the Web. This document instead
focuses on a non-Web-based environment and focuses on those protocols
where HTTP is not used. Despite the increased excitement for
layering every protocol on top of HTTP there are still a number of
protocols available that do not use HTTP-based transports. Many of
these protocols are lacking a native authentication and authorization
framework of the style shown in Figure 1.
1.6. Use of AAA
Interestingly, for network access authentication the usage of the AAA
framework with RADIUS [RFC2865] and Diameter [RFC3588] was quite
successful from a deployment point of view. To map the terminology
used in Figure 1 to the AAA framework the IdP corresponds to the AAA
server, the RP corresponds to the AAA client, and the technical
building blocks of a federation are AAA proxies, relays and redirect
agents (particularly if they are operated by third parties, such as
AAA brokers and clearing houses). The front-end, i.e. the end host
to AAA client communication, is in case of network access
authentication offered by link layer protocols that forward
authentication protocol exchanges back-and-forth. An example of a
large scale RADIUS-based federation is EDUROAM [2].
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Is it possible to design a system that builds on top of successful
protocols to offer non-Web-based protocols with a solid starting
point for authentication and authorization in a distributed system?
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Internet-Draft ABFAB Architecture July 20112. ArchitectureSection 1 already introduced the federated access architecture, with
the illustration of the different actors that need to interact, but
it did not expand on the specifics of providing support for non-Web
based applications. This section details this aspect and motivates
design decisions. The main theme of the work described in this
document is focused on re-using existing building blocks that have
been deployed already and to re-arrange them in a novel way.
Although this architecture assumes updates to both the relying party
as well as to the end host for application integration, those changes
are kept at a minimum. A mechanism that can demonstrate deployment
benefits (based on ease of update of existing software, low
implementation effort, etc.)is preferred and there may be a need to
specify multiple mechanisms to support the range of different
deployment scenarios.
There are a number of ways for encapsulating EAP into an application
protocol. For ease of integration with a wide range of non-Web based
application protocols the usage of the GSS-API was chosen.
Encapsulating EAP into the GSS-API also allows EAP to be used in
SASL. A description of the technical specification can be found in
[I-D.ietf-abfab-gss-eap]. Other alternatives exist as well and may
be considered later, such as "TLS using EAP Authentication"
[I-D.nir-tls-eap].
There are several architectural layers in the system; this section
discusses the individual layers.
2.1. Federation Substrate
The federation substrate is responsible for the connunication between
the relying party and the identity provider. This layer is
responsible for the inter-domain communication and for the technical
mechanisms necessary to establish inter-domain trust.
A key design goal is the re-use of an existing infrastructure, we
build upon the AAA framework as utilized by RADIUS [RFC2138] and
Diameter [RFC3588]. Since this document does not aim to re-describe
the AAA framework the interested reader is referred to [RFC2904].
Building on the AAA infrastructure, and RADIUS and Diameter as
protocols, modifications to that infrastructure is to be avoided.
Also, modifications to AAA servers should be kept at a minimum.
The astute reader will notice that RADIUS and Diameter have
substantially similar characteristics. Why not pick one? A key
difference is that today RADIUS is largely transported upon UDP, and
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its use is largely, though not exclusively, intra-domain. Diameter
itself was designed to scale to broader uses. We leave as a
deployment decision, which protocol will be appropriate.
Through the integrity protection mechanisms in the AAA framework, the
relying party can establish technical trust that messages are being
sent by the appropriate relying party. Any given interaction will be
associated with one federation at the policy level. The legal or
business relationship defines what statements the identity provider
is trusted to make and how these statements are interpreted by the
relying party. The AAA framework also permits the relying party or
elements between the relying party and identity provider to make
statements about the relying party.
The AAA framework provides transport for attributes. Statements made
about the subject by the identity provider, statements made about the
relying party and other information is transported as attributes.
2.1.1. Discovery, Rules Determination, and Technical Trust
One demand that the AAA substrate must make of the upper layers is
that they must properly identify the end points of the communication.
That is- it must be possible for the AAA client at the RP to
determine where to send each RADIUS or Diameter message. Without
this requirement, it would be the RP's responsibility to determine
the identity of the principal on its own, without the assistance of
an IdP. This architecture makes use of the Network Access Identifier
(NAI), where the IdP is indicated in the realm component [RFC4282].
The NAI is represented and consumed by the GSS-API layer as
GSS_C_NT_USER_NAME as specified in [RFC2743]. The GSS-API EAp
mechanism includes the NAI in the EAP Response/Identity message.
The RP needs to discover which federation will be used to contact the
IDP. As part of this process, the RP determines the set of business
rules and technical policies governing the relationship; this is
called rules determination. The RP also needs to establish technical
trust in the communications with the IDP.
Rules determination covers a broad range of decisions about the
exchange. One of these is whether the given RP is permitted to talk
to the IDP using a given federation at all, so rules determination
encompasses the basic authorization decision. Other factors are
included, such as what policies govern release of information about
the principal to the RP and what policies govern the RP's use of this
information. While rules determination is ultimately a business
function, it has significant impact on the technical exchanges. The
protocols need to communicate the result of authorization. When
multiple sets of rules are possible, the protocol must disambiguate
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which set of rules are in play. Some rules have technical
enforcement mechanisms; for example in some federations intermediates
validate information that is being communicated within the
federation.
Several deployment approaches are possible. These can most easily be
classified based on the mechanism for technical trust that is used.
The choice of technical trust mechanism constrains how rules
determination is implemented. Regardless of what deployment strategy
is chosen, it is important that the technical trust mechanism
constrain the names of both parties to the exchange. The trust
mechanism ought to ensure that the entity acting as IDP for a given
NAI is permitted to be the IDP for that realm, and that any service
name claimed by the RP is permitted to be claimed by that entity.
Here are the categories of technical trust determination:
AAA Proxy: The simplest model is that an RP supports a request
directly to an AAA proxy. The hop-by-hop integrity protection of
the AAA fabric provides technical trust. An RP can submit a
request directly to a federation. Alternatively, a federation
disambiguation fabric can be used. Such a fabric takes
information about what federations the RP is part of and what
federations the IDP is part of and routes a message to the
appropriate federation. The routing of messages across the fabric
plus attributes added to requests and responses provides rules
determination. For example, when a disambiguation fabric routes a
message to a given federation, that federation's rules are chosen.
Naming is enforced as messages travel across the fabric. The
entities near the RP confirm its identity and validate names it
claims. The fabric routes the message towards the appropriate
IDP, validating the IDP's name in the process. The routing can be
statically configured. Alternatively a routing protocol could be
developed to exchange reachability information about given IDPs
and to apply policy across the AAA fabric. Such a routing
protocol could flood naming constraints to the appropriate points
in the fabric.
Trust Broker: Instead of routing messages through AAA proxies, some
trust broker could establish keys between entities near the RP and
entities near the IDP. The advantage of this approach is
efficiency of message handling. Fewer entities are needed to be
involved for each message. Security may be improved by sending
individual messages over fewer hops. Rules determination involves
decisions made by trust brokers about what keys to grant. Also,
associated with each credential is context about rules and about
other aspects of technical trust including names that may be
claimed. A routing protocol similar to the one for AAA proxies is
likely to be useful to trust brokers in flooding rules and naming
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constraints.
Global Credential: A global credential such as a public key and
certificate in a public key infrastructure can be used to
establish technical trust. A directory or distributed database
such as the Domain Name System is needed for an RP to discover
what endpoint to contact for a given NAI. Certificates provide a
place to store information about rules determination and naming
constraints. Provided that no intermediates are required and that
the RP and IDP are sufficient to enforce and determine rules,
rules determination is reasonably simple. However applying
certain rules is likely to be quite complex. For example if
multiple sets of rules are possible between an IDP and RP,
confirming the correct set is used may be difficult. This is
particularly true if intermediates are involved in making the
decision. Also, to the extent that directory information needs to
be trusted, rules determination may be more complex.
Real-world deployments are likely to be mixtures of these basic
approaches. For example, it will be quite common for an RP to route
traffic to a AAA proxy within an organization. That proxy MAY use
any of the three methods to get closer to the IDP. It is also likely
that rather than being directly reachable, an IDP may have a proxy
within its organization. Federations MAY provide a traditional AAA
proxy interface even if they also provide another mechanism for
increased efficiency or security.
2.2. Subject To Identity Provider
Traditional web federation does not describe how a subject
communicates with an identity provider. As a result, this
communication is not standardized. There are several disadvantages
to this approach. It is difficult to have subjects that are machines
rather than humans that use some sort of programatic credential. In
addition, use of browsers for authentication restricts the deployment
of more secure forms of authentication beyond plaintext username and
password known by the server. In a number of cases the
authentication interface may be presented before the subject has
adequately validated they are talking to the intended server. By
giving control of the authentication interface to a potential
attacker, then the security of the system may be reduced and phishing
opportunities introduced.
As a result, it is desirable to choose some standardized approach for
communication between the subject's end-host and the identity
provider. There are a number of requirements this approach must
meet.
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Experience has taught us one key security and scalability
requirement: it is important that the relying party not get in
possession of the long-term secret of the entity being authenticated
by the AAA server. Aside from a valuable secret being exposed, a
synchronization problem can also often develop. Since there is no
single authentication mechanism that will be used everywhere there is
another associated requirement: The authentication framework must
allow for the flexible integration of authentication mechanisms. For
instance, some identity providers may require hardware tokens while
others may use passwords. A service provider would want to support
both sorts of federations, and others.
Fortunately, these requirements can be met by utilizing standardized
and successfully deployed technology, namely by the Extensible
Authentication Protocol (EAP) framework [RFC3748]. Figure 2
illustrates the integration graphically.
EAP is an end-to-end framework; it provides for two-way communication
between a peer (i.e,service client or principal) through the
authenticator (i.e., service provider) to the back-end (i.e.,
identity provider). Conveniently, this is precisely the
communication path that is needed for federated identity. Although
EAP support is already integrated in AAA systems (see [RFC3579] and
[RFC4072]) several challenges remain: one is to carry EAP payloads
from the end host to the relying party. Another is to verify
statements the relying party has made to the subject, confirm these
statements are consistent with statements made to the identity
provider and confirm all the above are consistent with the federation
and any federation-specific policy or configuration. Another
challenge is choosing which identity provider to use for which
service.
2.3. Application to Service
One of the remaining layers is responsible for integration of
federated authentication into the application. There are a number of
approaches that applications have adopted for security. So, there
may need to be multiple strategies for integration of federated
authentication into applications. However, we have started with a
strategy that provides integration to a large number of application
protocols.
Many applications such as SSH [RFC4462], NFS [RFC2203], DNS [RFC3645]
and several non-IETF applications support the Generic Security
Services Application Programming Interface [RFC2743]. Many
applications such as IMAP, SMTP, XMPP and LDAP support e Simple
Authentication and Security Layer (SASL) [RFC4422] framework. These
two approaches work together nicely: by creating a GSS-API mechanism,
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SASL integration is also addressed. In effect, using a GSS-API
mechanism with SASL simply requires placing some headers on the front
of the mechanism and constraining certain GSS-API options.
GSS-API is specified in terms of an abstract set of operations which
can be mapped into a programming language to form an API. When
people are first introduced to GSS-API, they focus on it as an API.
However, from the prospective of authentication for non-web
applications, GSS-API should be thought of as a protocol not an API.
It consists of some abstract operations such as the initial context
exchange, which includes two sub-operations (gss_init_sec_context and
gss_accept_sec_context). An application defines which abstract
operations it is going to use and where messages produced by these
operations fit into the application architecture. A GSS-API
mechanism will define what actual protocol messages result from that
abstract message for a given abstract operation. So, since this work
is focusing on a particular GSS-API mechanism, we generally focus on
protocol elements rather than the API view of GSS-API.
The API view has significant value. Since the abstract operations
are well defined, the set of information that a mechanism gets from
the application is well defined. Also, the set of assumptions the
application is permitted to make is generally well defined. As a
result, an application protocol that supports GSS-API or SASL is very
likely to be usable with a new approach to authentication including
this one with no required modifications. In some cases, support for
a new authentication mechanism has been added using plugin interfaces
to applications without the application being modified at all. Even
when modifications are required, they can often be limited to
supporting a new naming and authorization model. For example, this
work focuses on privacy; an application that assumes it will always
obtain an identifier for the principal will need to be modified to
support anonymity, unlinkability or pseudonymity.
So, we use GSS-API and SASL because a number of the application
protocols we wish to federate support these strategies for security
integration. What does this mean from a protocol standpoint and how
does this relate to other layers? This means we need to design a
concrete GSS-API mechanism. We have chosen to use a GSS-API
mechanism that encapsulates EAP authentication. So, GSS-API (and
SASL) encapsulate EAP between the end-host and the service. The AAA
framework encapsulates EAP between the relying party and the identity
provider. The GSS-API mechanism includes rules about how principals
and services are named as well as per-message security and other
facilities required by the applications we wish to support.
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Internet-Draft ABFAB Architecture July 20112.4. Personalization Layer
The AAA framework provides a way to transport statements from the
identity provider to the relying party. However, we also need to say
more about the content of these statements. In simple cases,
attributes particular to the AAA protocol can be defined. However in
more complicated situations it is strongly desirable to re-use an
existing protocol for asking questions and receiving information
about subjects. SAML is used for this.
SAML usage may be as simple as the identity provider including a SAML
Response message in the AAA response. Alternatively the relying
party may generate a SAML request XXX to whom, how, and at what
point? (see above XXX).
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Internet-Draft ABFAB Architecture July 20113. Application Security Services
One of the key goals is to integrate federated authentication into
existing application protocols and where possible, existing
implementations of these protocols. Another goal is to perform this
integration while meeting the best security practices of the
technologies used to perform the integration. This section describes
security services and properties required by the EAP GSS-API
mechanism in order to meet these goals. This information could be
viewed as specific to that mechanism. However, other future
application integration strategies are very likely to need similar
services. So, it is likely that these services will be expanded
across application integration strategies if new application
integration strategies are adopted.
3.1. Server (Mutual) Authentication
GSS-API provides an optional security service called mutual
authentication. This service means that in addition to the initiator
providing (potentially anonymous or pseudonymous) identity to the
acceptor, the acceptor confirms its identity to the initiator.
Especially for the ABFAB context, this service is confusingly named.
We still say that mutual authentication is provided when the identity
of an acceptor is strongly authenticated to an anonymous initiator.
RFC 2743 does not explicitly talk about what mutual authentication
means. Within the GSS-API community successful mutual authentication
has come to mean:
o If a target name is supplied by the initiator, then the initiator
trusts that the supplied target name describes the acceptor. This
implies both that appropriate cryptographic exchanges took place
for the initiator to make such a trust decision, and that after
evaluating the results of these exchanges, the initiator's policy
trusts that the target name is accurate.
o The initiator trusts that its idea of the acceptor name correctly
names the entity it is communicating with.
o Both the initiator and acceptor have the same key material for
per-message keys and both parties have confirmed they actually
have the key material. In EAP terms, there is a protected
indication of success.
Mutual authentication is an important defense against certain aspects
of phishing. Intuitively, users would like to assume that if some
party asks for their credentials as part of authentication,
successfully gaining access to the resource means that they are
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talking to the expected party. Without mutual authentication, the
acceptor could "grant access" regardless of what credentials are
supplied. Mutual authentication better matches this user intuition.
It is important, therefore, that the GSS-EAP mechanism implement
mutual authentication. That is, an initiator needs to be able to
request mutual authentication. When mutual authentication is
requested, only EAP methods capabale of providing the necessary
service can be used, and appropriate steps need to be taken to
provide mutual authentication. A broader set of EAP methods could be
supported when a particular application does not request mutual
authentication. It is an open question whether the mechanism will
permit this.
3.2. GSS-API Channel Binding
[RFC5056] defines a concept of channel binding to prevent man-in-the-
middle attacks. It is common to provide SASL and GSS-API with
another layer to provide transport security; Transport Layer Security
(TLS) is the most common such layer. TLS provides its own server
authentication. However there are a variety of situations where this
authentication is not checked for policy or usability reasons. Even
when it is checked, if the trust infrastructure behind the TLS
authentication is different from the trust infrastructure behind the
GSS-API mutual authentication. If the endpoints of the GSS-API
authentication are different than the endpoints of the lower layer,
this is a strong indication of a problem such as a man-in-the-middle
attack. Channel binding provides a facility to determine whether
these endpoints are the same.
The GSS-EAP mechanism needs to support channel binding. When an
application provides channel binding data, the mechanism needs to
confirm this is the same on both sides consistent with the GSS-API
specification. XXXThere is an open question here as to the details;
today RFC 5554 governs. We could use that and the current draft
assumes we will. However in Beijing we became aware of some changes
to these details that would make life much better for GSS
authentication of HTTP. We should resolve this with kitten and
replace this note with a reference to the spec we're actually
following.
Typically when considering channel binding, people think of channel
binding in combination with mutual authentication. This is
sufficiently common that without additional qualification channel
binding should be assumed to imply mutual authentication. Without
mutual authentication, only one party knows that the endpoints are
correct. That's sometimes useful. Consider for example a user who
wishes to access a protected resource from a shared whiteboard in a
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conference room. The whiteboard is the initiator; it does not need
to actually authenticate that it is talking to the correct resource
because the user will be able to recognize whether the displayed
content is correct. If channel binding were used without mutual
authentication, it would in effect be a request to only disclose the
resource in the context of a particular channel. Such an
authentication would be similar in concept to a holder-of-key SAML
assertion. However, also note that while it is not happening in the
protocol, mutual authentication is happening in the overall system:
the user is able to visually authenticate the content. This is
consistent with all uses of channel binding without protocol level
mutual authentication found so far.
RFC 5056 channel binding (also called GSS-API channel binding when
GSS-API is involved) is not the same thing as EAP channel binding.
EAP channel binding is also used in the ABFAB context in order to
implement acceptor naming and mutual authentication. Details are
discussed in the mechanisms specification [I-D.ietf-abfab-gss-eap].
3.3. Host-Based Service Names
IETF security mechanisms typically take the name of a service entered
by a user and make some trust decision about whether the remote party
in an interaction is the intended party. GSS-API has a relatively
flexible naming architecture. However most of the IETF applications
that use GSS-API, including SSH, NFS, IMAP, LDAP and XMPP, have
chosen to use host-based service names when they use GSS-API. In
this model, the initiator names an acceptor based on a service such
as "imap" or "host" (for login services such as SSH) and a host name.
Using host-based service names leads to a challenging trust
delegation problem. Who is allowed to decide whether a particular
hostname maps to an entity. The public-key infrastructure (PKI) used
by the web has chosen to have a number of trust anchors (root
certificate authorities) each of wich can map any name to a public
key. A number of GSS-API mechanisms suchs as Kerberos [RFC1964]
split the problem into two parts. A new concept called a realm is
introduced. Then the mechanism decides what realm is responsible for
a given name. That realm is responsible for deciding if the acceptor
entity is allowed to claim the name. ABFAB needs to adopt this
approach.
Host-based service names do not work ideally when different instances
of a service are running on different ports. Also, these do not work
ideally when SRV record or other insecure referrals are used.
The GSS-EAP mechanism needs to support host-based service names in
order to work with existing IETF protocols.
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Internet-Draft ABFAB Architecture July 20113.4. Per-Message Tokens
GSS-API provides per-message security services that can provide
confidentiality and integrity. Some IETF protocols such as NFS and
SSH take advantage of these services. As a result GSS-EAP needs to
support these services. As with mutual authentication, per-message
services will limit the set of EAP methods that are available. Any
method that produces a Master Session Key (MSK) should be able to
support per-message security services.
GSS-API provides a pseudo-random function. While the pseudo-random
function does not involve sending data over the wire, it provides an
algorithm that both the initiator and acceptor can run in order to
arrive at the same key value. This is useful for designs where a
successful authentication is used to key some other function. This
is similar in concept to the TLS extractor. No current IETF
protocols require this. However GSS-EAP supports this service
because it is valuable for the future and easy to do given per-
message services. Non-IETF protocols are expected to take advantage
of this in the near future.
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Internet-Draft ABFAB Architecture July 20114. Future Work: Attribute Providers
This architecture provides for a federated authentication and
authorization framework between IdPs, RPs, principals, and subjects.
It does not at this time provide for a means to retrieve attributes
from 3rd parties. However, it envisions such a possibility. We note
that in any extension to the model, an attribute provider must be
authorized to release specific attributes to a specific RP for a
specific principal. In addition, we note that it is an open question
beyond this architecture as to how the RP should know to trust a
particular attribute provider.
There are a number of possible technical means to provide attribute
provider capabilities. One possible approach is for the IdP to
provide a signed attribute request to RP that it in turn will provide
to the attribute authority. Another approach would be for the IdP to
provide a URI to the RP that contains a token of some form. The form
of communications between the IdP and attribute provider as well as
other considerations are left for the future. One thing we can say
now is that the IdP would merely be asserting who the attribute
authority is, and not the contents of what the attribute authority
would return. (Otherwise, the IdP might as well make the query to
the attribute authority and then resign it.)
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Internet-Draft ABFAB Architecture July 20115. Privacy Considerations
Sharing identity information raises privacy violations and as
described throughout this document an existing architecture is re-
used for a different usage environment. As such, a discussion about
the privacy properties has to take these pre-conditions into
consideration. We use the approach suggested in
[I-D.morris-privacy-considerations] to shed light into what data is
collected and used by which entity, what the relationship between
these entities and the end user is, what data about the user is
likely needed to be collected, and what the identification level of
the data is.
5.1. What entities collect and use data?
Figure 2 shows the architecture with the involved entities. Message
exchanges are exchanged between the client application, via the
relying part to the AAA server. There will likely be intermediaries
between the relying party and the AAA server, labeled generically as
"federation".
In order for the relying party to route messages to the AAA server it
is necessary for the client application to provide enough information
to enable the identification of the AAA server's domain. While often
the username is attached to the domain (in the form of a Network
Access Identity (NAI) this is not necessary for the actual protocol
operation. The EAP server component within the AAA server needs to
authenticate the user and therefore needs to execute the respective
authentication protocol. Once the authentication exchange is
complete authorization information needs to be conveyed to the
relying party to grant the user the necessary application rights.
Information about resource consumption may be delivered as part of
the accounting exchange during the lifetime of the granted
application session.
The authentication exchange may reveal an identifier that can be
linked to a user. Additionally, a sequence of authentication
protocol exchanges may be linked together. Depending on the chosen
authentication protocol information at varying degrees may be
revealed to all parties along the communication path. This
authorization information exchange may disclose identity information
about the user. While accounting information is created by the
relying party it is likely to needed by intermediaries in the
federation for financial settlement purposes and will be stored for
billing, fraud detection, statistical purposes, and for service
improvement by the AAA server operator.
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Internet-Draft ABFAB Architecture July 20115.2. Relationship between User's and other Entities
The AAA server is a first-party site the user typically has a
relationship with. This relationship will be created at the time
when the security credentials are exchange and provisioned. The
relying party and potential intermediares in the federation are
typically operate under the contract of the first-party site. The
user typically does not know about the intermediaries in the
federation nor does he have any relationship with them. The protocol
interaction triggered by the client application happens with the
relying party at the time of application access. The relying party
does not have a direct contractual relationship with the user but
depending on the application the interaction may expose the brand of
the application running by the relying party to the end user via some
user interface.
5.3. What Data about the User is likely Needed to be Collected?
The data that is likely going to be collected as part of a protocol
exchange with application access at the relying party is accounting
information and authorization information. This information is
likely to be kept beyond the terminated application usage for trouble
shooting, statistical purposes, etc. There is also like to be
additional data collected to to improve application service usage by
the relying party that is not conveyed to the AAA server as part of
the accounting stream.
5.4. What is the Identification Level of the Data?
With regard to identification there are several protocol layers that
need to be considered separately. First, there is the EAP method
exchange, which as an authentication and key exchange protocol has
properties related to identification and protocol linkage. Second,
there is identification at the EAP layer for routing of messages.
Then, in the exchange between the client application and the relying
party the identification depends on the underlying application layer
protocol the EAP/GSS-API exchange is tunneled over. Finally, there
is the backend exchange via the AAA infrastructure, which involves a
range of RADIUS and Diameter extensions and yet to be defined
extensions, such as encoding authorization information inside SAML
assertions.
Since this document does not attempt to define any of these exchanges
but rather re-uses existing mechanisms the level of identification
heavily depends on the selected mechanisms. The following two
examples aim to illustrate the amount of existing work with respect
to decrease exposure of personal data.
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1. When designing EAP methods a number of different requirements may
need to get considered; some of them are conflicting. RFC 4017
[RFC4017], for example, tried to list requirements for EAP
methods utilized for network access over Wireless LANs. It also
recommends the end-user identity hiding requirement, which is
privacy-relevant. Some EAP methods, such as EAP-IKEv2 [RFC5106],
also fulfill this requirement.
2. EAP, as the layer encapsulating EAP method specific information,
needs identity information to allow routing requests towards the
user's home AAA server. This information is carried within the
Network Access Identifier (NAI) and the username part of the NAI
(as supported by RFC 4282 [RFC4282]) can be obfuscated.
5.5. Privacy Challenges
While a lot of standarization work was done to avoid leakage of
identity information to intermediaries (such as eavesdroppers on the
communication path between the client application and the relying
party) in the area of authentication and key exchange protocols.
However, from current deployments the weak aspects with respect to
security are:
1. Today business contracts are used to create federations between
identity providers and relying parties. These contracts are not
only financial agreements but they also define the rules about
what information is exchanged between the AAA server and the
relying party and the potential involvement of AAA proxies and
brokers as intermediaries. While these contracts are openly
available for university federations they are not public in case
of commercial deployments. Quite naturally, there is a lack of
transparency for external parties.
2. In today's deployments the ability for users to determine the
amount of information exchanged with other parties over time, as
well as the possibility to control the amount of information
exposed via an explict consent is limited. This is partially due
the nature of application capabilities at the time of network
access authentication. However, with the envisioned extension of
the usage, as described in this document, it is desirable to
offer these capabilities.
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Internet-Draft ABFAB Architecture July 20117. Security Considerations
This entire document is about security. A future version of the
document will highlight some important security concepts.
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Internet-Draft ABFAB Architecture July 20119. Acknowledgments
We would like to thank Mayutan Arumaithurai and Klaas Wierenga for
their feedback. Additionally, we would like to thank Eve Maler,
Nicolas Williams, Bob Morgan, Scott Cantor, Jim Fenton, and Luke
Howard for their feedback on the federation terminology question.
Furthermore, we would like to thank Klaas Wierenga for his review of
the pre-00 draft version.
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